Issue with validatequotes

### This issue is for a:
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> cd sgx.attest.sample.intel.sdk/validatequotes.core
> ./runone.sh

### Any log messages given by the failure
```
dnat@myvm:~/microsoft-azure-attestation/sgx.attest.sample.intel.sdk/validatequotes.core$ ./runone.sh
[04:42:16.901] :
[04:42:16.923] : ************************************************************************************************************************
[04:42:16.923] : * PARAMETERS FOR THIS RUN
[04:42:16.923] : ************************************************************************************************************************
[04:42:16.923] : Validating filename : ../genquotes/out/enclave.info.prodid.json
[04:42:16.923] : Using attestation provider : sharedcus.cus.attest.azure.net
[04:42:16.923] : Including details : True
** Message: 04:42:17.094: Remote error from secret service: org.freedesktop.DBus.Error.UnknownMethod: No such interface 'org.freedesktop.Secret.Collection' on object at path /org/freedesktop/secrets/collection/login
Unhandled exception. System.AggregateException: One or more errors occurred. (SharedTokenCacheCredential authentication failed: Persistence check failed. Data was written but it could not be read. Possible cause: on Linux, LibSecret is installed but D-Bus isn't running because it cannot be started over SSH.)
---> Azure.Identity.AuthenticationFailedException: SharedTokenCacheCredential authentication failed: Persistence check failed. Data was written but it could not be read. Possible cause: on Linux, LibSecret is installed but D-Bus isn't running because it cannot be started over SSH.
---> Microsoft.Identity.Client.Extensions.Msal.MsalCachePersistenceException: Persistence check failed. Data was written but it could not be read. Possible cause: on Linux, LibSecret is installed but D-Bus isn't running because it cannot be started over SSH.
at Microsoft.Identity.Client.Extensions.Msal.MsalCacheStorage.VerifyPersistence()
at Microsoft.Identity.Client.Extensions.Msal.MsalCacheHelper.VerifyPersistence()
at Azure.Identity.MsalClientBase`1.GetClientAsync(Boolean async, CancellationToken cancellationToken)
at Azure.Identity.MsalClientBase`1.GetClientAsync(Boolean async, CancellationToken cancellationToken)
at Azure.Identity.MsalPublicClient.GetAccountsAsync(Boolean async, CancellationToken cancellationToken)
at Azure.Identity.SharedTokenCacheCredential.GetAccountAsync(Boolean async, CancellationToken cancellationToken)
at Azure.Identity.SharedTokenCacheCredential.GetTokenImplAsync(Boolean async, TokenRequestContext requestContext, CancellationToken cancellationToken)
--- End of inner exception stack trace ---
at Azure.Identity.CredentialDiagnosticScope.FailWrapAndThrow(Exception ex)
at Azure.Identity.SharedTokenCacheCredential.GetTokenImplAsync(Boolean async, TokenRequestContext requestContext, CancellationToken cancellationToken)
at Azure.Identity.SharedTokenCacheCredential.GetTokenAsync(TokenRequestContext requestContext, CancellationToken cancellationToken)
at Azure.Identity.DefaultAzureCredential.GetTokenFromSourcesAsync(TokenCredential[] sources, TokenRequestContext requestContext, Boolean async, CancellationToken cancellationToken)
at Azure.Identity.DefaultAzureCredential.GetTokenImplAsync(Boolean async, TokenRequestContext requestContext, CancellationToken cancellationToken)
at Azure.Identity.CredentialDiagnosticScope.FailWrapAndThrow(Exception ex)
at Azure.Identity.DefaultAzureCredential.GetTokenImplAsync(Boolean async, TokenRequestContext requestContext, CancellationToken cancellationToken)
at Azure.Identity.DefaultAzureCredential.GetTokenAsync(TokenRequestContext requestContext, CancellationToken cancellationToken)
at Azure.Core.Pipeline.BearerTokenAuthenticationPolicy.AccessTokenCache.GetHeaderValueFromCredentialAsync(TokenRequestContext context, Boolean async, CancellationToken cancellationToken)
at Azure.Core.Pipeline.BearerTokenAuthenticationPolicy.AccessTokenCache.GetHeaderValueAsync(HttpMessage message, TokenRequestContext context, Boolean async)
at Azure.Core.Pipeline.BearerTokenAuthenticationPolicy.AccessTokenCache.GetHeaderValueAsync(HttpMessage message, TokenRequestContext context, Boolean async)
at Azure.Core.Pipeline.BearerTokenAuthenticationPolicy.AuthenticateAndAuthorizeRequestAsync(HttpMessage message, TokenRequestContext context)
at Azure.Core.Pipeline.BearerTokenAuthenticationPolicy.ProcessAsync(HttpMessage message, ReadOnlyMemory`1 pipeline, Boolean async)
at Azure.Core.Pipeline.RetryPolicy.ProcessAsync(HttpMessage message, ReadOnlyMemory`1 pipeline, Boolean async)
at Azure.Core.Pipeline.RetryPolicy.ProcessAsync(HttpMessage message, ReadOnlyMemory`1 pipeline, Boolean async)
at Azure.Security.Attestation.AttestationRestClient.AttestSgxEnclaveAsync(AttestSgxEnclaveRequest request, CancellationToken cancellationToken)
at Azure.Security.Attestation.AttestationClient.AttestSgxEnclaveInternal(AttestationRequest request, Boolean async, CancellationToken cancellationToken)
at Azure.Security.Attestation.AttestationClient.AttestSgxEnclaveAsync(AttestationRequest request, CancellationToken cancellationToken)
at validatequotes.Program.RunAsync() in /home/dnat/microsoft-azure-attestation/sgx.attest.sample.intel.sdk/validatequotes.core/Program.cs:line 79
--- End of inner exception stack trace ---
at System.Threading.Tasks.Task.WaitAllCore(Task[] tasks, Int32 millisecondsTimeout, CancellationToken cancellationToken)
at System.Threading.Tasks.Task.WaitAll(Task[] tasks)
at validatequotes.Program.Main(String[] args) in /home/dnat/microsoft-azure-attestation/sgx.attest.sample.intel.sdk/validatequotes.core/Program.cs:line 19
```

### OS and Version?
> Ubuntu 18.04

### Mention any other details that might be useful

I am trying to run the intel sdk attestation example on an Azure DC8 v2 instance. The genquotes portion seems to work. However, I am getting a DBus error during the validatequotes portion (when calling `maaService.AttestSgxEnclaveAsync`).

I am also a little unclear about how access to “default” azure attestation providers (like `sharedcus.cus.attest.azure.net`) is supposed to work. Is access to services provided by these attestation providers automatic and guaranteed (assuming I have an Azure subscription), or do I need to configure something in my subscription to be able to use any of these default services? If so, could this be the issue? Thanks!

Start with sgx.attest.sample.intel.sdk/validatequotes.core/runone.sh and the stack-trace locations in Program.cs at lines 79 and 19, using the Ubuntu 18.04 environment described. Investigate whether the DBus/LibSecret persistence failure or access to sharedcus.cus.attest.azure.net prevents the AttestSgxEnclaveAsync call from completing. Done means the validatequotes example runs successfully in the reported environment or documents the confirmed prerequisite and failure cause.

Written by the indexing model from the issue text.